Schmidt et al., U.S. Pat. No. 3,697,471, disclose a family of ester imide resins made by reacting together at least one polybasic acid or a functional derivative thereof, and at least one polyhydric alcohol or functional derivative thereof, at least one of the reactants having at least one five-membered imide ring between the functional groups of the molecule. It further discloses that the reactants can be heated in a commercial cresol mixture, then further diluted in a mixture of naphtha and cresol and used as an enamel for coating copper wire to produce a hard, thermally resistant insulation therefor. Meyer et al., U.S. Pat. No. 3,426,098, describe ester imide resins in which all or part of the polyhydric alcohol comprises tris(2-hydroxyethyl) isocyanurate.
Keating, U.S. Pat. Nos. 4,119,605, 4,119,608; and 4,119,758, incorporated herein by reference, discloses low molecular weight polyester-imide resins which are soluble in non-phenolic solvents by incorporating a monoether of a diethylene glycol or a triethylene glycol into the resin. Keating states that the use of the monoether of a glycol as a reactant serves to control the molecular weight and solubility of the polymer and permits the manufacture of wire enamels which contain relatively non-toxic solvents in place of phenols and also permits the attainment of enamels having economical high solids contents. Boldebuck, Banucci and Byrne in commonly-assigned copending application Ser. No. 867,939, filed Jan. 9, 1978, disclose a way of preparing virtually solvent-free polyesterimides, suitable for hot melt coating, by adding to the reaction mixture a monofunctional alcohol-reactant, such as decyl alcohol or a glycol ether.
Sheffer et al., U.S. Pat. Nos. 2,982,754, and Meyer et al., 3,211,585, describe cresylic acid-solubilized polyester-polyisocyanates for use as wire enamels.
Zamek, in commonly-assigned copending application Ser. No. 100,212, filed Dec. 4, 1979, describes in Example 5 the production of a partially completely reacted polyesterimide (acid number 6-7) and its reduction by a glycol ether solvent prior to reaction in solution with a blocked polyisocyanate to produce a coating composition which is an extremely viscous liquid at room temperature and has a solids content of 67.72%. It is stated to be adapted to be hot applied to copper wire at 80.degree. C. The foregoing patents and applications are incorporated herein by reference.
It has now been discovered that virtually solventless reaction products of polyesterimides and blocked isocyanates can be made for use as wire enamels applied from the melt. As mentioned above, typically, wires are coated with enamels that are solutions of from 20 to 50% solids in cresylic acid and non-cresylic acid solvent systems. Application of those enamels require that the solvents be removed before cure and subsequently burned or released into the atmosphere. This procedure involves substantial use of energy and has environmental shortcomings. The present invention avoids these problems.
The hot melt polyesterimide component is produced herein by polymerization of the reactants at extremely high solids content (&gt;75%) and direct isolation of the reaction product with blocked polyisocyanate at the end of the reaction. An important aspect of the invention is the production of a resin with the appropriate viscosity at the various hot melt applicator temperatures and the ability to add the necessary curing additives, e.g., the blocked polyisocyanate, to the polyester imide component at high temperature without the aid of a solvent. In the present invention, the use of a minimum amount of a glycol ether or ester, such as the monomethyl ether of diethylene glycol or the mono-n-butyl ether of diethylene glycol as a reaction mixture component aids in processing, e.g., the removal of volatile by-products and temperature control, and such monofunctional compounds function as a chain stopper limiting molecular weight and thus high temperature viscosity. The blocked polyisocyanates are introduced at higher than normally expected temperatures, even at 180.degree. C. and, surprisingly, no premature cure is observed. In any case, it is preferred to employ a conventional transesterification catalyst, such as a titanate, and this can be added all in the beginning or some at the beginning and the balance at the end.